Field of the Invention
This invention relates, generally, to a catalyst system. Specifically, this invention relates to hemiisospecific catalysts.
Description of Related Art
Olefins, especially propylene, may be polymerized to form polyolefins in various forms: isotactic, syndiotactic and atactic. Isotactic polypropylene contains principally repeating units with identical configurations and only a few erratic, brief inversions in the chain. Isotactic polypropylene may be structurally represented in a Fischer projection as ##STR1## In Bovey's NMR nomenclature the isotactic structure is designated . . . mmmm . . . since the five successive methyl groups are meso to each other, i.e., on the same side of the plane in a Fischer projection.
Isotactic polypropylene is capable of being a highly crystalline polymer with a high melting point and other desirable physical properties that are considerably different from the polymer in an amorphous (noncrystalline) state.
A syndiotactic polymer contains principally units of exactly alternating stereoisomers and is represented in a Fischer projection by the structure: ##STR2## In Bovey's NMR nomenclature the syndiotactic structure is designated . . . rrrr . . . since the five successive methyl groups are racemic to each other, i.e., on alternate sides of the plane in a Fischer projection.
A polymer chain showing no regular order of repeating unit configurations is an atactic polymer. In commercial applications, a certain percentage of atactic polymer is typically produced with the isotactic form.
There are other variations in the form of polymer structure. Hemiisotactic or hemitactic polypropylene was disclosed in "Hemitactic Polypropylene: An Example of a Novel Kind of Polymer Tacticity" by M. Farina, G. Di Silvestro and P. Sozzani (Macromolecules, Vol. 15, 1451-1452, 1982). The structure of hemiisotactic polymers is represented in a Fischer projection as follows: ##STR3##
The monomeric unit of the polymer is of the following structure: ##STR4## where R.sub.s is a hydrocarbyl group or nonhydrocarbyl group. The second carbon atom in formula (6) is the asymmetric carbon atom, i.e., the one which does not have identical groups attached, hence "asymmetric".
The structure of the polymer is characterized by R.sub.s groups attached to every other asymmetric carbon atom being on the same side of the principal polymer chain as represented in a Fischer projection and R.sub.s groups attached to the remaining asymmetric carbon atoms being either on the same side or the opposite side of the R.sub.s groups attached to every other asymmetric carbon atom. When R.sub.s groups are on the same side of the principal polymer chain, the structure is isotactic. Since only every other one conforms to the isotactic structure, it is "hemi". The material is a noncrystalline polymer. Polymerization of olefins is primarily with Zeigler-Natta catalysts. One family of Zeigler-Natta catalysts is Group IV metallocene compounds with methylaluminoxane as a cocatalyst. German patent application No. 2,608,863 discloses a catalyst system for the polymerization of ethylene consisting of bis(cyclopentadienyl)titanium dialkyl, an aluminum trialkyl and water. German patent application No. 2,608,933 discloses an ethylene polymerization catalyst system consisting of zirconium metallocenes of the general formula (cyclopentadienyl).sub.n ZrY.sub.4-n, wherein Y represents R.sub.1 CH.sub.2 AlR.sub.2, CH.sub.2 CH.sub.2 AlR.sub.2 and CH.sub.2 CH(AlR.sub.2).sub.2 where R stands for an alkyl or metallo alkyl, and n is a number within the range 1- 4; and the metallocene catalyst is used in combination with an aluminum trialkyl cocatalyst and water.
The use of metallocenes as catalysts in the copolymerization of ethylene and other alpha-olefins is also known in the art. U.S. Pat. No. 4,542,199 to Kaminsky, et al. discloses a process for the polymerization of olefins and particularly for the preparation of polyethylene and copolymers of polyethylene and other alpha-olefins. The disclosed catalyst system includes a catalyst of the formula (cyclopentadienyl).sub.2 MeRHal in which R is a halogen, a cyclopentadienyl or a C.sub.1 -C.sub.6 alkyl radical, Me is a transition metal, in particular zirconium, and Hal is a halogen, in particular chlorine. The catalyst system also includes an alumoxane having the general formula Al.sub.2 OR.sub.4 (Al(R)-O)n for a linear molecule and/or (Al(R)-O).sub.n+2 for a cyclic molecule in which n is a number from 4-20 and R is a methyl or ethyl radical. A similar catalyst system is disclosed in U.S. Pat. No. 4,404,344.
U.S. Pat. No. 4,530,914 discloses a catalyst system for the polymerization of ethylene to polyethylene having a broad molecular weight distribution and especially a bimodal or multimodal molecular weight distribution. The catalyst system is comprised of at least two different metallocenes and an alumoxane. The patent discloses metallocenes that may have a bridge between two cyclopentadienyl rings with the bridge serving to make those rings stereorigid.
European Patent Publication No. 0185918 discloses a stereorigid, chiral zirconium metallocene catalyst for the polymerization of olefins. The application does not indicate that hafnium could be substituted for the zirconium and used to produce a useful polymer product. The bridge between the cyclopentadienyl groups is disclosed as being a linear hydrocarbon with 1-4 carbon atoms or a cyclical hydrocarbon with 3-6 carbon atoms. European Patent Application 0-277-003 relates to work by Turner on a catalyst prepared by a protonation method. A bis(cyclopentadienyl) metal compound is combined with a compound having a cation capable of donating a proton and an anion having a plurality of boron atoms. For example, the following reaction illustrates the invention: ##STR5## where [B] is 7,8-dicarbaundecaborane. European Patent Application 0-277-004 also relates to work by Turner on a catalyst prepared by a protonation method. A bis(cyclopentadienyl) metal compound is combined with an ionic compound having a cation which will irreversibly react with a ligand on the metal compound and an anion having a plurality of lipophilic radicals around a metal or metalloid ion. For example, the following reaction illustrates the invention:
A system for the production of isotactic polypropylene using a titanium or zirconium metallocene catalyst and an alumoxane cocatalyst is described in "Mechanisms of Stereochemical Control in Propylene Polymerization with Soluble Group 4B Metallocene/Methylalumoxane Catalysts," J. Am. Chem. Soc., Vol. 106, pp. 6355-64, 1984. The article shows that chiral catalysts derived from the racemic enantiomers of ethylene-bridged indenyl derivatives form isotactic polypropylene by the conventional structure predicted by an enantiomorphic-site stereochemical control model. The meso achiral form of the ethylene-bridged titanium indenyl diastereomers and achiral zirconocene derivatives, however, produce polypropylene with a purely atactic structure.